Traditional Chinese Simplified Chinese

  • Knowing more about Very Hot Weather Warning

  • Wednesday, 24th August 2016

After more than a week of rainy days, the weather in Hong Kong finally turned fine but became rather hot this week. We all know that the Observatory will issue warning in case of very hot weather. However, you may recall there was occasion when the warning was in force, the maximum temperature recorded at the Observatory throughout the day did not reach 33 degrees. Why is this the case?

Figure 1

The Observatory will issue warning when the weather is very hot

As a matter of fact, the issuance of Very Hot Weather Warning is not only based on the temperature recorded at the Observatory headquarters in Kowloon, but also those recorded in the weather stations on the Hong Kong Island, the New Territories and outlying island, as well as the Hong Kong Heat Index which represents the combined effect of temperature, humidity, wind speed and solar radiation. Generally speaking, when the index reaches around 30, it means that the heat may cause adverse health effects. Taking 3 June this year as an example, the maximum temperature recorded at the Observatory was 32.4 degrees, but the Hong Kong Heat Index reached 30.9 (see diagram below) owing to abundant sunshine, moderate winds and slightly humid weather. The Very Hot Weather Warning was in force that day and the actual weather did meet the criterion of warning.

Figure 2

Hong Kong Heat Index recorded on 3 June 2016

Checking the records since 2014, we found that among all days with the Very Hot Weather Warning in effect, the actual weather met the warning criterion on around 80 percent of the days. Such performance is considered good as far as weather forecast is concerned. It is worth noting that the Observatory also provides Hot Weather Special Advisory, the issuance of which is based on basically the same factors as the Very Hot Weather Warning but at a different degree. It is usually issued when the weather conditions do not warrant the issuance of Very Hot Weather Warning but the hot weather might cause adverse effects to the health of the public.

When you engage in outdoor activities in summer, please take note of the Very Hot Weather Warning or Hot Weather Special Advisory and take precautions to avoid heat stroke or sunburn.

L.S. Lee    Y.H. Chan

  • Uncertainty in genesis of tropical cyclone

  • Thursday, 18th August 2016

Since midweek last week, the Observatory has anticipated the formation of tropical cyclones over a stretch of area from the northern part of the South China Sea to the western North Pacific this week, yet the formation location was rather uncertain at that time. Why is there such an uncertainty? To start with, we need to talk about the favourable conditions for the genesis of tropical cyclone.

One of the prerequisites for tropical cyclone formation is the presence of cyclonic convergence (anti-clockwise flow of airstream in the Northern Hemisphere) at the lower levels of atmosphere. One of the systems that are favourable for tropical cyclone formation is the Intertropical Convergence Zone, where the southwesterly airstream flowing from the Southern Hemisphere to the Northern Hemisphere meets the northeasterly airstream at the southern periphery of the subtropical high in the Northern Hemisphere. The convergence zone favours the development of cyclonic airflows and convective activities. The satellite imagery (Figure 1) this Tuesday (16 August 2016) noon depicts a rather long band of convective clouds extending from the northern part of the South China Sea to the western North Pacific, indicating the position of the Intertropical Convergence Zone.

Figure 1

Figure 1      Satellite imagery at 12 noon on 16 August 2016 with yellow arrows indicating the direction of movement of airstreams. (Source of satellite imagery: Japan Meteorological Agency)

Although different computer prediction models had already forecast earlier the formation of a tropical cyclone in the Intertropical Convergence Zone this week, there were differences between various models on when and where the cyclones would form along the convergence zone, which spanned a few thousands of kilometres. This added uncertainty to the forecast. In fact, with reference to the radar imagery captured at 6 am on Tuesday (Figure 2), a broad area of low pressure appeared over the south China coast and the northern part of the South China Sea, where there were more than one centres of cyclonic flow or low pressure. Some of them existed for only a while and dissipated. As to which one could finally develop into a tropical cyclone would depend on various factors such as the sea surface temperature, transport of moisture, vertical variations of winds, and divergence of airflow at the upper atmosphere. The Observatory kept monitoring and assessing the development of this broad area of low pressure. Eventually a centre of cyclonic flow developed at about 200 kilometres southwest of Hong Kong over the coastal waters of western Guangdong. It subsequently intensified into a tropical depression on Wednesday morning (17 August), warranting the issuance of tropical cyclone warning signal by the Observatory.

Figure 2

Figure 2      Radar imagery at 6 am on 16 August 2016 with the symbol "L" indicating the positions of the centres of low pressure.

Indeed, whenever the Intertropical Convergence Zone or a broad trough of low pressure is close to Hong Kong, no matter whether a tropical cyclone will ultimately formed, there will be chances of squally showers, thunderstorms and swells in Hong Kong. The public is therefore advised to keep track of the latest weather information from the Observatory. Those engaging in water sports or staying near the shore should take extra care against the danger brought by the rough seas.

C.Y.Y. Leung and L.S. Lee

  • Behind the Global Glacial Meltdown

  • Monday, 15th August 2016

Ice Age is a long period of low temperature conditions with glaciers covering many parts of the Earth. The period between Ice Ages is called "interglacial" which has a milder climate. The last Ice Age ended about 10,000 years ago. Since then, the Earth has entered into an interglacial period with a warmer climate, becoming more inhabitable for humans and favourable for civilization to flourish.

At the end of the last Ice Age, glaciers around the world started to melt and retreat. Owing to the uncertainty in the estimation of timing, it was difficult to tell for sure whether the glacier retreats in different parts of the world were synchronous. As such, the glacial meltdown was also attributed to regional factors such as changes in solar radiation, precipitation pattern and ocean circulation.

Scientists have recently re-examined more than 1000 boulders left behind by the retreating glaciers across the globe and applied an improved methodology to estimate the timing of glacier retreats[1,2]. The boulders, previously covered by the glaciers, would be exposed to the atmosphere and bombarded by cosmic rays, producing the radioactive isotope Beryllium-10. By analysing the level of the isotope, scientists were able to estimate the exposure age of the boulders and hence the time when the glaciers retreated. The results revealed that the retreat of glaciers across the globe was broadly synchronous with the rise of atmospheric concentration of carbon dioxide as derived from ice core analyses. Using climate model simulations, it was concluded that the primary cause of the global glacial meltdown was the rise of atmospheric concentration of carbon dioxide (Figure 1).

Figure 1

Figure 1      Scientists concluded that the primary cause of the global glacial meltdown at the end of the last Ice Age was the rise of atmospheric concentration of carbon dioxide. (Photo courtesy: Oregon State University)

It took about 7000 years for the concentration of carbon dioxide to rise from 180 ppm to 280 ppm near the end of the last Ice Age. Atmospheric concentration of carbon dioxide has been increasing since the Industrial Revolution, and it only took a century or so for the concentration to exceed 400 ppm (Figure 2). With the concentration of carbon dioxide keeps setting new records, should we not be worried about the accelerated melting of glaciers around the world and the subsequent sea level rise (Figure 3)?

Figure 2

Figure 2      Variation of atmospheric carbon dioxide concentration over the last 800,000 years. (Source: World Meteorological Organization)

Figure 3

Figure 3      Accelerated glacier retreat. (Source: "CHASING ICE")

F.C. Sham


[1] As Ice Age ended, greenhouse gas rise was lead factor in melting of Earth's glaciers.

[2] Regional and global forcing of glacier retreat during the last deglaciation.